1. Field of the Invention
The present invention relates to a catalyst for the removal of nitrogen oxides from a waste gas and a process for the production thereof, in which the end of the catalyst precursor on the gas-inlet side is coated with a biphosphate, a condensed phosphate a silicate, a borate or a mixture thereof, then the coated catalyst precursor is subjected to a thermal aftertreatment. The present invention also concerns a process for removing nitrogen oxides from a waste gas, using the catalyst.
2. Discussion of the Background
The removal of nitrogen oxides from waste gases is of general importance in many industrial processes. For example, the burning of fossil fuels in power stations produces nitrogen oxides, which pass into the environment together with the flue gases.
In order to remove the nitrogen oxides from flue gases by selective catalytic reduction using ammonia as reducing agent, German Patent 2,458,888 employs catalysts based on high surface-area, porous titanium dioxides with further metal oxide or sulphate components. The catalyst types employed for the removal of nitrogen oxides have a multiplicity of parallel channels, the axes of which are aligned in the direction of the gas flow. The catalysts used are predominantly (1) monolithic with a honeycomb structure (see German Patent 2,658,569 and German Patent 2,658,539), or (2) catalyst packets comprising a multiplicity of individual plate-like structures (see German Patent 3,208,634, German Patent 2,853,023 and German Patent 2,927,253).
These catalysts are frequently operated in "high-dust" circuits, preferably at temperatures between 350.degree. and 450.degree. C. (see German Patent 2,658,539 and German Patent 2,834,358). The catalytic reactor in a high-dust circuit of this type is positioned upstream of a dust filter. The flue gas at this point contains a particularly large proportion of fly ash or fly dust, so that the catalyst for the removal of nitrogen oxides is subjected to considerable abrasion caused by fly ash, predominantly from the flow side. This significantly reduces the service life of the catalyst.
JP-A 79/71791 discloses a fixed-bed catalyst, provided with through-holes in the gas flow direction. At least one front end of the catalyst on the gas-inlet side is coated with a substance whose principal component comprises biphosphate, condensed phosphate or metaphosphate, and is heated in order to reinforce this.
According to Example 1 of JP-A 79/71791, a honeycomb element is shaped from a titanium dioxide-based catalyst composition by extrusion. The shaped element is dried, then calcined at 500.degree. C. After this production process, one end (i.e., the later flow side of the catalyst) is subsequently immersed into an aqueous preparation of aluminum biphosphate, and the catalyst is re-dried and re-calcined at 500.degree. C.
The additional application of a layer of this type, which reinforces the catalyst structure on the flow side, significantly improves the abrasion resistance and the service life of the catalyst under high-dust conditions. However, experience has shown that the process for the production of a catalyst according to JP-A 79/71791 is time-consuming and expensive. Furthermore, although direct incorporation of the reinforcing component (for example, aluminum biphosphate) into the catalyst composition before shaping allows an improvement in the catalyst strength, the catalytic activity of the entire catalyst drops significantly at the same time.